The invention relates to immersion lithography apparatus and methods, and particularly to apparatus and methods for inhibiting immersion liquid from flowing (for example, due to wicking) to the under-surface of a substrate that is being exposed on a substrate stage.
A typical lithography apparatus includes a radiation source, a projection optical system and a substrate stage to support and move a substrate to be imaged. A radiation-sensitive material, such as a resist, is coated onto the substrate surface before the substrate is placed on the substrate stage. During operation, radiation energy from the radiation source is used to project an image defined by an imaging element through the projection optical system onto the substrate. The projection optical system typically includes a plurality of lenses. The lens or optical element closest to the substrate can be referred to as the last or final optical element.
The projection area during exposure is typically much smaller than the imaging surface of the substrate. The substrate therefore is moved relative to the projection optical system in order to pattern the entire surface of the substrate. In the semiconductor industry, two types of lithography apparatus are commonly used. With so-called “step-and-repeat” apparatus, the entire image pattern is projected at one moment in a single exposure onto a target area of the substrate. After the exposure, the substrate is moved or “stepped” in the X and/or Y direction(s) and a new target area is exposed. This step-and-repeat process is performed multiple times until the entire substrate surface is exposed. With scanning type lithography apparatus, the target area is exposed in a continuous or “scanning” motion. For example, when the image is projected by transmitting light through a reticle or mask, the reticle or mask is moved in one direction while the substrate is moved in either the same or the opposite direction during exposure of one target area. The substrate is then moved in the X and/or Y direction(s) to the next scanned target area. The process is repeated until all of the desired target areas on the substrate have been exposed.
Lithography apparatus are typically used to image or pattern semiconductor wafers and flat panel displays. The word “substrate” as used herein is intended to generically mean any work piece that can be patterned including, but not limited to, semiconductor wafers and flat panel displays.
Immersion lithography is a technique that can enhance the resolution of lithography exposure apparatus by permitting exposure to take place with a numerical aperture (NA) that is greater than the NA that can be achieved in conventional “dry” lithography exposure apparatus. By filling the space between the final optical element of the projection system and the resist-coated substrate, immersion lithography permits exposure with light that would otherwise be internally reflected at the optic-air interface. Numerical apertures as high as the index of the immersion fluid (or of the resist or lens material, whichever is least) are possible in immersion lithography systems. Liquid immersion also increases the substrate depth-of-focus, that is, the tolerable error in the vertical position of the substrate, by the index of the immersion fluid compared to a dry system having the same numerical aperture. Immersion lithography thus can provide resolution enhancement equivalent to a shift from 248 nm to 193 nm without actually decreasing the exposure light wavelength. Thus, unlike a shift in the exposure light wavelength, the use of immersion would not require the development of new light sources, optical materials (for the illumination and projection systems) or coatings, and can allow the use of the same or similar resists as conventional “dry” lithography at the same wavelength. In an immersion system in which only the final optical element of the projection system and its housing and the substrate (and perhaps portions of the stage as well) are in contact with the immersion fluid, much of the technology and design developed for dry lithography can carry over directly to immersion lithography.
However, because the substrate moves rapidly in a typical lithography system, the immersion fluid in the immersion area between the projection system and the substrate tends to be carried away from the immersion area. If the immersion fluid escapes from the immersion area, that fluid can interfere with operation of other components of the lithography system. One way to recover the immersion fluid and prevent the immersion fluid from contaminating the immersion lithography system is described in US2006/0152697 A1, the disclosure of which is incorporated herein by reference in its entirety. US2006/0152697 A1 discloses an immersion fluid supply and recovery system in which a porous member surrounds the immersion area and is in fluid communication with the space that defines the immersion area. The porous member is maintained at a pressure that is under the bubble point of the porous member, such that immersion fluid that escapes from the immersion area is captured (recovered) by the porous member. The porous member encircles the immersion area and is maintained at a substantially constant low pressure.
Another problem that exists in immersion lithography apparatus is the undesired flow of the immersion liquid to the under-surface of the substrate that is being exposed. Immersion liquid that was not recovered from the substrate can move to the edge of the substrate, for example, due to movement of the substrate by the substrate stage, and then flow to the under-surface of the substrate. Moreover, even with systems in which a localized area is provided with immersion liquid, such as the systems described in the above-identified US2006/0152697A1, the localized immersion area extends beyond the periphery of the substrate when exposure takes place near the edge of the substrate. It is known to make the portion of the substrate stage surrounding the periphery of the substrate substantially flush with the upper surface of the substrate and to dispose the surrounding stage portion very close to the substrate periphery in order to inhibit the flow of immersion liquid over the substrate periphery and to the under-surface of the substrate. However, it is not uncommon for some immersion liquid to flow or wick (that is move by capillary action) through the small gap between the substrate periphery and the surrounding portion of the substrate stage, and thus wet the under-surface of the substrate.
It is undesirable to wet the under-surface of the substrate because that may cause the substrate to stick to the substrate holding member, making it difficult to remove the substrate from the substrate holding member when exposure is completed. Additionally, immersion liquid that flows to the under-surface of the substrate can enter the vacuum passages that are used to hold the substrate to the substrate holding member, which is not desirable. Other undesirable effects of the liquid not being directed away from the substrate or its moving assembly include liquid damage to motors that move the substrate, and liquid interfering with substrate stage position sensors, which could cause a system crash.